Insertion at the base of the fifth metacarpal is a characteristic of which muscle?

• A. Flexor Carpi Ulnaris
• B. Extensor Digitorum
• C. Extensor Carpi Ulnaris
• D. Extensor Carpi Radialis Longus

Answer: (C)

Where a muscle attaches helps reveal its function at the wrist. Inserting at the base of the fifth metacarpal on the dorsal side is the hallmark location for the extensor carpi ulnaris. This tendon reaches the base of the fifth metacarpal as it runs along the ulnar side of the forearm, so the muscle’s main actions are to extend the wrist and tilt it toward the ulna (ulnar deviation). The other muscles either attach to different locations or serve different primary actions: for example, extensor digitorum acts on the fingers, extensor carpi radialis longus attaches near the base of the second metacarpal, and flexor carpi ulnaris attaches on the palmar side for wrist flexion. The insertion pattern and the resulting wrist-extending, ulnar-deviating action point to the extensor carpi ulnaris.